(1) Field
This disclosure relates to an optical device. This disclosure more particularly relates to an optical device with modulators used in optical communication.
(2) Description of the Related Art
In the field of optical communication, optical devices with a Mach-Zehnder interference optical modulator have been developing (see, for example, Japanese Unexamined Patent Publication No. 2008-46573).
Such a modulator included in an optical device is formed by, for example, forming an optical waveguide in an electro-optic crystal substrate and locating electrodes near the optical waveguide. The optical waveguide of the modulator can broadly be divided into an input waveguide where light is input and propagated, a pair of modulation waveguides where the light propagated through the input waveguide is split and propagated, and an output waveguide where the light propagated through the pair of modulation waveguides is combined, propagated, and output. A signal electrode and an earth electrode are located over the pair of modulation waveguides. A method for locating lumped-constant type electrodes or traveling-wave type electrodes as electrodes of a modulator is known. For example, if traveling-wave type electrodes are located, then an end of the signal electrode and an end of the earth electrode are connected via a resistor and a microwave signal is applied from the input side. At this time the refractive index of each of the pair of modulation waveguides changes and the phase of light which is input to the input waveguide and which is propagated through the pair of modulation waveguides changes. Accordingly, intensity-modulated signal light is output from the output waveguide because of Mach-Zehnder interference.
An optical sending apparatus for polarization multiplex communication which includes two modulators each having the above structure is proposed. The polarization modes of signal light output from the optical sending apparatus are, for example, TM mode and TE mode. That is to say, the polarization modes of signal light output from the optical sending apparatus are perpendicular to each other. For example, the following structure is proposed. In order to obtain signal light the polarization modes of which are perpendicular to each other, light output from one of the two modulators is made to pass through a transmission λ/2 plate. In addition, in order to multiplex the signal light the polarization modes of which are perpendicular to each other, a polarization beam coupler (PBC) is located on the end side of two output waveguides.
In addition to a transmission plate, a reflection plate is proposed as a component, such as a λ/2 plate, for converting a polarization mode in the field of optical communication (see, for example, Japanese Unexamined Patent Publication No. 08-278422).
In order to miniaturize the optical device including the above modulators, it is desirable that the two modulators and the PBC should be formed in one substrate for the purpose of forming them in one chip. Accordingly, it is necessary to locate a component, such as the λ/2 plate, for rotating polarization in the chip.
For example, the method of forming a groove in a portion of an output waveguide included in one of the two modulators for cutting the output waveguide and of inserting the λ/2 plate into the groove is known as a method for locating the λ/2 plate in the chip. With this method, however, a plurality of optical device chips are formed on one wafer (electro-optic crystal substrate) and the wafer is cut into the plurality of optical device chips. After that, the above process must be performed on each chip. This requires a large number of steps. In addition, a production yield may deteriorate because of, for example, damage to a chip caused by the formation of the groove. Furthermore, if this method is adopted, a great optical loss occurs in the groove where the λ/2 plate is located, or there is variation in optical loss among chips.